1. Field of the Invention
This invention relates generally to electrical safety modules and, more particularly, to an electrical protector device which safeguards telecommunications circuits from excessive voltage and excessive current and, moreover, is adapted to provide electrical test accessing.
2. Description of the Prior Art
In modern telecommunication systems, protection apparatus safeguards personnel and equipment against overvoltage and overcurrent conditions. For example, in telephone central offices, the protection apparatus typically comprises a protector panel with protection devices inserted therein. The panel is usually located at the juncture between outside plant cable facilities and central office equipment and each protection device serves the two-fold purpose of electrically interconnecting the outside facilities to inside equipment and monitoring for abnormal electrical conditions at the juncture. In addition, the panel oftentimes incorporates a test point field so that electrical test access to outside facilities or inside equipment may be conveniently established.
The protector device normally utilizes series protection for excessive currents and shunt protection for excessive voltages. Series protection is usually effected by a pair of heat sensitive modules generally referred to as heat coils. Each coil is inserted in series with an incoming line and is responsive to undesired current which may damage facilities or equipment if allowed to persist. Excessive heat generated by the unwanted, persistent current flow on the line activates means for diverting the current through a path to ground.
Shunt protection is usually afforded by symmetrical pairs of axially aligned, ring-shaped carbon blocks which are arranged to define a so-called spark gap between them. One block from a pair is electrically grounded whereas the other is electrically connected to one wire of the incoming line. Excessive voltage caused by, for instance, lightning, is bypassed to ground by arcing within the spark gap.
U.S. Pat. No. 3,573,695, issued to J. B. Geyer et al on Apr. 6, 1971, is representative of prior art provisioning devices that combine carbon block and heat coil protection within a single module. The heat coil assembly includes a conductive pin which permanently contacts one wire of the incoming line. During excessive current flow, heat melts a solder joint holding the pin in place and allows a coil spring to drive the pin into contact with the grounded carbon block, thus diverting current through a resistive path to ground.
While devices of this type have provided generally satisfactory protection against overload conditions, they do exhibit shortcomings. One shortcoming is that intense, localized heat may be generated within the module after the pin completes the resistive path to ground. This heat may be sufficient to distort or even melt certain plastic elements in the device, thereby hampering protection effectiveness. A further disadvantage is that the pin reaches the grounded carbon block by penetrating a hole in the second carbon block. Particles may accumulate in the spark gap by dropping through the hole and result in shorting of the spark gap. Another shortcoming is that the heat coil assembly requires inordinate space within the limited volume of the module and, accordingly, additional features are precluded from incorporation within the device.
The protector modules disclosed in U.S. Pat. Nos. 4,004,192 and 4,004,263, both issued to W. V. Carney on Jan. 18, 1977, reduce excessive heat generation by diverting unwanted current through a totally metallic path to ground. In Carney '192, heat releases a coil spring which drives and then holds a conductive plunger in simultaneous contact with both the incoming line and a grounded metallic projection. In Carney '263, a coil spring drives an elongated, metallic bar protruding from the conductive plunger directly into contact with a flat ground plate. A compressible, conductive member insures electrical connectivity between the plunger and the incoming line irrespective of the plunger displacement.
In either of these designs, an aperture in one of the carbon blocks forming the spark gap guides axial motion of the plunger. Again, particle accumulation within the narrow spark gap may inhibit normal operation. In addition to this deficiency, the placement of the delicate heat coil external to the coil spring enhances the possibility of damage to the heat coil during assembly.
Another recent example of a protector design is disclosed in U.S. Pat. No. 4,057,692, issued to G. DeBortoli et al on Nov. 8, 1977. Although DeBortoli et al relate generally to protector panel apparatus, a protector device used in conjunction with the protector panel is also disclosed. This device is very similar to the previously mentioned designs, particularly with respect to the spark gap arrangement. In addition, in order to divert excessive current directly to ground, a coil spring drives a pin through an opening in a ring-shaped insulative member so as to contact ground. Particle accumulation within the opening in the ring member as well as within the spark gap represent shortcomings of this design.
The subject matter of U.S. Pat. No. 4,215,381, issued to R. F. Heisinger on July 29, 1980, and assigned to the same assignee as the present application, discloses a protector device that mitigates many of the shortcomings of the aforementioned devices. For example, the nongrounded carbon block no longer has an aperture, so problems caused by tiny particles accumulating within the spark gap are diminished. Also, the heat coil pin is stationary and remains directly in contact with both the nongrounded carbon block and the incoming line. This eliminates the need for additional internal space to permit pin movement. Furthermore, the heat coil assembly is within the helix of the coil spring. This arrangement virtually eliminates any potential damage to the delicate heat coil wire during assembly. Finally, the carbon blocks are enclosed in a metallic shield which permanently contacts a ground plate. This arrangement enhances heat dissipation by increasing the area of heated surfaces exposed to cooling effects. However, as elaborated upon later, an improvement in the device of the instant application provides for enhanced heat dissipation characteristics by enlarging the area of the ground assembly in permanent contact with the shield. This improvement mitigates regions of concentrated heat and virtually precludes distorting effects on plastic members, including the protector housing.
As alluded to earlier, access to a plurality of telephone pairs for testing is conventionally accomplished via a field of test points which are electrically bridged to the panel appearance of the pairs. This stand-alone test field appearance makes inefficient use of available space within a central office. The protector device disclosed herein eliminates the necessity of an independent appearance of the test field by providing internal test lands accessible with external probing means.
U.S. Pat. No. 4,012,096, issued to DeLuca et al on Mar. 15, 1977, is the closest art which relates to the instant application with respect to internal test lands. In DeLuca et al, a combined connector-test field panel is disclosed for effecting interconnection of outside plant cables to inside equipment. Both the incoming pairs and central office equipment appear side by side on pin couplets protruding from one face of the panel. Shorting plugs inserted on this face jumper adjacent couplets. It is suggested in DeLuca et al that the shorting plug housings may include apertures through which electrical probes may be inserted to check potentials directly at the connection point. A most severe limitation with this arrangement is the absence of protection. Moreover, protected testing is precluded. Furthermore, since the plug has no intermediate seating position, outside plant pairs may not be isolated from inside equipment and, accordingly, protected testing solely into the outside plant environment is impossible.